A glitch filter is used in computer systems to sample an electrical signal (i.e., the input signal to the filter) and output the glitch-free state of the signal. In particular, a glitch filter allows a sampled signal to change its state (e.g., from a LOW state to a HIGH state, or from a HIGH state to a LOW state) only after it exhibits this change in state for a certain period of time. A glitch filter may be implemented using either analog or digital circuitry. The present invention relates to the latter.
Typically, a digital glitch filter includes two stages of sampling and a comparison between the two stages. Each stage of sampling may be implemented by using a flip-flop, and the comparison may be implemented by using logic gates. The output signal of the glitch filter may change only if the input signal remains in the changed state for two consecutive system clock pulses.
Nineteen gates are required to implement such a glitch filter in TSB140G, a 1.0 .mu.m cmos technology. When a large number of signals are to be filtered, the nineteen gates required per filter may lead to large total space requirements as required by the larger die area of the prior art design and high costs.